net: allow sh_eth to get mac address through platform data

Extend the sh_eth driver to allow passing the mac address
using the platform data structure. This to simplify board
setup code.

Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>